Process of removing coatings from nickel and cobalt base refractory alloys



July 29, 1969 A. L. BALDI 3,458,353

PROCESS OF REMOVING COATINGS FROM NICKEL AND COBALT BASE REFRACTORY ALLOYS Filed Nov. 16, 1966 -oooe" -PwA 663 Cr IN Ni 0R Cr BASE REFRACTORY v- H WEIGHT LOSS IMMERSED IN l2- HNO |O% HF 2% N H 0 7 88% 5 .0 115 E FOR 30min. U E

I g a 0004 LLI 3 E 4 -o'oo2" -Udimef 779 I 2 --ooo| Udlmef 50o "r'rg x ooooz' Rene 4| O l l l 1 0 4 8 I2 I6 20 24 28 32 36 4O WEIGHT "/0 Cr.

IN VEN TOR. i/fowu L. 5 4/4! ATTO RNEYS United States Patent 3,458,353 PROCESS OF REMOVING COATINGS FROM NICKEL AND COBALT BASE REFRACTORY ALLOYS Alfonso L. Baldi, Drexel Hill, Pa., assignor to Alloy Surfaces Company, Inc., Wilmington, DeL, a corporation of Delaware Filed Nov. 16, 1966, Ser. No. 594,757 Int. Cl. B08b 3/08 US. Cl. 1343 4 Claims ABSTRACT OF THE DISCLOSURE Stripping diffused coatings of chromium-aluminum or of aluminum from alloys having a base of more than 50% by weight of nickel or of cobalt or of a mixture thereof and which contain at least 18% of chromium by weight. The coating is treated with a water solution containing /2 to 10% by weight of hydrofluoric acid and preferably 1 to 3%, with between 5 and 20% by weight of nitric acid, preferably between 7 and 15%, and most desirably between 8 and 10%. The treatment is preferably employed at a temperature between 140 F. and the boiling temperature, and most desirably at a temperature of between 170 and 200 F.

The present invention relates to a process of stripping diffused coatings of chromium-aluminum or of aluminum ice A further purpose is to remove chromium-aluminum and aluminum diffused coatings from such refractory alloys by a process which is less obnoxious, dangerous and toxic than the hydrochloric acid baths previously used.

A further purpose is to shorten the time required for stripping chromium-aluminum and aluminum diffused coatings from nickel or cobalt base refractory alloys containing at least 18% by weight of chromium.

Further purposes appear in the specification and in the claims.

The drawing shows a curve useful in explaining the invention, plotting loss in weight in milligrams per square centimeter as ordinate against weight percentage of chromium in the refractory alloy as abscissa.

Extensive use is being made in the art of so-ca1led refractory alloys having a base of nickel or cobalt (by this it is meant that more than 50% by weight of the alloy consists either of nickel or cobalt or a mixture of nickel and cobalt) and containing various amounts of some of the following: chromium, carbon, imolybdenum, tungsten, columbium, titanium, aluminum, iron, boron, manganese, copper, zirconium, tantalum, silicon or other alloying additions.

All of these alloys under discussion besides a base of nickel or cobalt contain significant amounts of chromium, as mentioned more in detail 'below.

The following table gives representative analyses by Weight for numerous alloys of this character which are used in the art.

0 Cr Ni B Others Udi at 500 O. 08 19. 0 Bal. X-40 25. 0 10. 0 Rene 41. 0.09 19. 0 B21. Hastelloy X 0. 10 22. 0 Bal. L 605 (AMS5759) 0.13 19. 35 10.48

PWA 653 (WI-52) 0. 21. 0 1. 0 PWA 657 (SM 302) 0.93 21. 5 T

9.0 $81-15 O8 12 B31. 15 6. 5 1. 5 0. 5 2. 5 5. 5 5 02 gll 1 PWA 663 8. 0 Ba]. 10.0 6.0 1.0 4.3 Ta PWA 659 (SM 200) 0. 17 9. 0 Hal. 10. 0 12. 5 1. 0 2. 0 5. 0 02 .08 Zr PWA 689 (Udirnet 799) l5. 0 Ba]. 18. 5 8 3. 5 4. 5 0. 05 .005 Zr from refractory alloys having a base of nickel or of cobalt and containing at least 18% of chromium by weight.

A purpose of the invention is to avoid damage to refractory alloys having a nickel or cobalt base and having at least 18% by weight of chromium as a result of stripping diffused coatings of chromium-aluminum or of aluminum, and especially avoiding pitting and intergranular corrosion which may impair the surface of the parts.

-A further purpose is to more completely remove These refractory alloys are used for a wide variety of purposes, especially heat resisting service, one of which is blades, vanes, buckets and the like for gas turbines and jet engine parts. Unfortunately, although they have good properties at elevated temperature, many of these alloys are subject to corrosion in gas atmospheres or the like. In order to protect against this difficulty, diffusion coatings have been developed of chromium-aluminum or of aluminum. See Galmiche Nouveau Procd de Chromage Thermique et Formation dAlliages Mixtes de Diffusion, 47 Revue de Metallurgie (No. 3, 1950) 192; Galmiche,

chromium-aluminum and aluminum diffused coatings French Academy of Sciences (Jan. 2, 1950) 89; Galmiche,

from such nickel and cobalt base refractory alloys.

French Academy of Sciences (Mar. 20, 1950) 1181;

Galmiche, Nouveau Procd Chromage Thermique et Formation dAlliages Mixtes de Diffusion, La Rescherche Aeronautics (No. 14, 1950) 55. This process involves diffusion into the surface of the alloy of a mixture of chromium and aluminum or of aluminum from a pack normally consisting of metallic chromium and metallic aluminum or of metallic aluminum, plus a. suitable halide such as ammonium chloride and an inert material such as powdered alumina, the pack being either in contact with the refractory alloy parts or out of contact with them, the atmosphere being such as to exclude air and prevent oxidation and the temperature being suitably elevated.

Many times after utilization of such parts consisting of a nickel or cobalt base alloy, it is necessary to again coat the parts in order to further protect them against corrosion at elevated temperature or other deterioration. As a preliminary it is usually necessary to strip the whole coating from the surface.

The practice in the past in removing chromium-aluminum or aluminum diffused coatings from nickel or cobalt tbase alloys containing substantial amounts of chromium has been to pickle for a substantial time at elevated temperature with inhibited concentrated hydrochloric acid.

The usual routine followed in stripping chromium-aluminum or aluminum diffusion cases from alloy PWA 653 (WI-52) which contains 21% of chromium and has a cobalt base is as follows:

(1) Vapor degreasing with a vapor degreasing solvent.

(2) Sand blasting dry or wet.

(3) Pickling for one hour in concentrated technical grade hydrochloric acid plus an inhibitor at 150 to 160 F.

(4) Washing and drying.

(5) Eliminating the pickling esidue by wet sand blastmg.

(6) If necessary, acid pickling in the same acid again for 30 minutes followed by washing and then wet sand blasting.

This prior art stripping technique has been subject to serious objection as follows:

(1) It usually results in incomplete removal of the coating and repeated applications of pickling treatment are necessary.

(2) Intergranular corrosion occurs producing pits on the surface which may result in rejection of the parts, or of poor performance because of a non-uniform surface contour.

(3) The hydrochloric acid fumes, especially in this highly concentrated form, are quite obnoxious and the acid itself is toxic to workers.

(4) The treatment is very time consuming, requiring something more than an hour.

An effort has been made by the present inventor to find a process of stripping chromium-aluminum and aluminum diffused coatings from refractory alloys having a base of nickel or cobalt or a mixture thereof. A markedly improved process has been developed which is applicable only to such alloys containing at least 18% of chromium, but is not applicable to alloys of lower chromium content as later explained.

The stripping bath according to the invention is a water solution of hydrofluoric acid and nitric acid. This combination of etchants is not in itself broadly new, but as employed in the present invention offers very unusual advantages of which the following are particularly significant.

(1) Unlike the hydrochloric acid bath above referred to, the above etching solution as applied to nickel or chromium base alloys containing at least 18% of chromium has negligible attack on the base metal, and confines its attack to the coating of chromium-aluminum or aluminum. In particular it does not subject the base metal to intergranular attack and does not cause pitting.

When reference is made herein to negligible attack, it is meant that no more than 2 mg./sq. cm. of untreated base refractory alloy will be removed by a solution of 2% of hydrofluoric acid and 10% of nitric acid by weight in water at 180 within 30 minutes and not more than 4 mg./sq. cm. of untreated base refractory alloy will be removed by the same solution at boiling temperature in 30 minutes. 2 mg./sq. cm. represents a loss of about 0.0001 inch reduction in thickness and 4 mg./sq. cm. loss represents a reduction in thickness of about 0.0002 inch.

(2) The new stripping composition is very much more effective in removing diffused coatings of chromium-aluminum and aluminum and it is very unlikely that the process will have to be repeated.

(3) The time required by the process of the present invention is much shorter than the preceding commercial procedure.

(4) The acid concentrations in the new stripping solution are so much lower that the fumes are much less obnoxious and the danger of toxicity is much less than in the case of concentrated hydrochloric acid.

It should be remembered that the technique of the present invention is not applicable to nickel base or chromium base alloys containing less than 18% of chro mium, and if attempts are made to strip chromium-aluminum or aluminum diffusion coatings from such alloys, there is serious danger of localized attack such as intergranular corrosion which will make the parts thus stripped unuseable. The figure shows for various alloys having various chromium contents and having a base of nickel or chromium or a mixture thereof, the effect in loss in weight plotted against chromium content in weight percent. It will be noted that as the chromium content lowers below 18% by weight the loss in weight increases startlingly, thus indicating that it would be highly unwise to apply this stripping solution to such alloys. The experiments shown in the drawing were made with a solution of 10% by weight of nitric acid, 2% by weight of hydrofluoric acid and 88% by weight of water at 175 F., the exposure time being 30 minutes.

Thus the last four alloys to the left listed in the table would be ruled out and should not be stripped by this solution.

It has been found that effective results in stripping chromium-aluminum or aluminum diffused coatings from nickel or cobalt base refractory alloys containing at least 18% by weight of chromium are obtained with concentrations of /2 to 10% by weight of hydrofluoric acid, preferably 1 to 3%, and with 5 to 20% by weight of nitric acid, preferably 7 to 15%, and most desirably between 8 and 10%, at temperatures of between F. and boiling, preferably between F. and 200 F. The treatment for about 15 minutes is sufficient to dissolve the coating or make it partially dissolve and partially flake off with negligible attack on the base refractory alloys.

Under these conditions the maximum metal removal from the base metal does not exceed about 4 mg./sq. cm. (0.002 inch) and in the preferred concentration range and preferred temperature range, the base metal removal in 15 minutes does not exceed about 2 mg./sq. cm.

Detailed procedure The preferred detailed procedure for stripping nickel or chromium base refractory alloys containing 18% or greater of chromium and having diffused chromiumaluminum or aluminum coatings is as follows:

(1) Vapor degreasing with a suitable solvent such as trichloroethylene.

(2) If the parts have previously been exposed to high temperature use and have a heat formed oxide on top of the diffused case, they should be sand blasted dry or wet. If there is no heat formed oxide, this preliminary blasting is unnecessary. It will be evident, however, that if the case produced is not of the desired thickness or quality,

it can be removed by the process of the invention without the necessity of prior sand blasting.

(3) The parts are immersed in a water solution, preferably containing 1 to 3% by weight of hydrofluoric acid and 8 to by weight of nitric acid, the balance being water at a temperature of 170 to 200 F. for a time of about minutes.

(4) The parts are rinsed throughly in water and then dried.

(5) The parts are then blasted with grit alumina to remove the residue from pickling.

While in the ordinary case it is sufficient to go through the procedure once in order to obtain adequate stripping, it will be evident that in exceptional cases, as for example where there are relatively inaccessible openings in the part, it may be necessary to go through the process repeatedly or to repeat certain steps.

The stripping process of the invention has been applied experimentally in reworking of vanes and blades of gas turbine jet engines. The parts after stripping were completely free from the coating, and no pitting or other intergranular corrosion occured. The fact that the solution was much less obnoxious and toxic was very beneficial to the personnel involved.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art to obtain all or part of the benefits of my invention without copying the process shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A process for stripping a diffused coating of chromium-aluminum or aluminum from a refractory alloy having more than 50 percent by weight of the class consisting of nickel, cobalt and mixtures thereof, and containing at least 18% of chromium by weight, which comprises treating a diffused coating of the class consisting of chromium-aluminum and aluminum on said refractory alloy with a water solution which essentially consists of /2 to 10% by weight of hydrofluoric acid, 5 to 20% by weight of nitric acid, and water.

2. A process of claim 1, which comprises maintaining the solution at a temperature of between F. and the boiling temperature.

3. A process of claim 1, in which the hydrofluoric acid is in a concentration range of between 1 and 3% by weight and the nitric acid is in a concentration range of between 7 and 15% by weight.

4. A process of claim 3, which comprises maintaining the solution at a temperature between and 200 F.

References Cited UNITED STATES PATENTS MORRIS O. WOLK, Primary Examiner M. D. BURNS, Assistant Examiner U.S. Cl. X.R.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 58,353 Dated July 29, 1969 Invencor(s) Alfonso L. Baldi It: is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

0013mm 1, line 59, "0.002" should read 0.0002" SIGNED AND $EALED MAR 01970 (SEAL) Attest:

Edward WILLIAM E saEuYLER .m Attesting Officer Oomissioner or Pat eats 

